The SAW device comprises a pair of electro-SAW transducers and a SAW propagation path established therebetween. In each transducer, parallel stripe electrodes are formed on a polished piezoelectric substrate so as to effect conversion between an electric signal and a SAW signal. Usually, the SAW propagation path is contituted by part of the polished piezoelectric substrate which is used in common for the transducers.
An acoustic wave that has frequencies as high as several gigahertz travels on the optically polished surface of a piezoelectric substrate at a velocity which is only about 10.sup.-5 times that of electromagnetic waves. A surface acoustic wave thus has the slow-travel property of sound while retaining the microwave frequency of its source. Saw devices utilize these properties and include a variety of delay lines, filters, pulse processors, and other microwave devices and circuits. In a SAW delay line, the delay is determined by the distance traveled by a surface acoustic wave on a piezoelectric surface.
In a SAW filter, the frequency characteristics of the filter are determinable mainly dependent on the velocity (sound velocity) of the SAW and an electrode pattern of the input/output transducers.
The electrode pattern of the transducer includes a plurality of parallel stripe electrodes. The orientation of the stripes determines the direction of a wave front. In a single phase transducer, all the stripe electrodes are maintained at a single phase, and one electrode stripe and one free area stripe (a gap region between adjacent electrodes) define a region for one wavelength. In an interdigital transducer, electrode stripes are formed at an interval of 1/2 wavelength, and two-phase control is carried out. More particularly, two comb shaped electrodes each having a plurality of electrode fingers are opposed and interlocked and they are maintained at opposite phases. Two electrode stripes and two free area stripes, four stripes in total, define a region for one wavelength. Typically, all of the stripes have the same width l which is .lambda./4, where .lambda. represents a wavelength of a SAW. For general knowledge of the SAW, reference should be made to IEEE. Trans, MTT-17, 11 (1969), "Special Issue on Microwave Acoustics". A proposal has been reported in which the finger widths of one electrode in an interdigital structure are enlarged in order to improve electrical, SAW reflective and the like characteristics of a transducer (".pi. MHz Band Low Loss SAW Filters" The Institute of Electronics and Communication Engineers of Japan, US 81-23, CPM 81-21, p33, July 20, 1981).
The stripe width needs to be decreased in proportion to a desired increase in the frequency. In an interdigital transducer for frequencies of several hundreds megahertz or more, the stripe width is often prescribed to be 5 .mu.m or less. In order to precisely set a center frequency for pass band of a filter within such a high frequency range, a pattern precision of submicron or less is required. When preparing an electrode pattern through typical photolithographic technology, the pattern precision is limited to some extent. For example, on account of the fact that the line width of a photomask pattern can be adjusted (increased or decreased) in steps of 0.1 .mu.m, the frequency cannot be controlled by a fraction of 1/10 or less of a center frequency when the pattern line width is 1 .mu.m.
With a center frequency of 800 MHz, an adjacent center frequency will be displaced therefrom by as large an amount as 80 MHz, thus preventing efficient utilization of the frequency range. Conversely, when it is desired to utilize the current UHF band, SAW devices fail to cover the entire range.
Electron beam exposure techniques can assure more precise adjustment of the pattern line width, but it disadvantageously requires complicated and expensive mask preparation processes.
Under the circumstances, the advent of SAW devices in which fine adjustment of the center frequency can be accomplished without resort to a highly precise electrode pattern has been needed urgently.